Extremely heavy loads such as metal castings, possibly weighing several tons, occasionally have to be moved around factory premises on wheeled trolleys. It is not always possible to pick up the load on a crane or overhead conveyor to move it from one part of the factory site to another. Load-moving tractor units are therefore available for pulling or pushing a heavy load on a wheeled trolley to different parts of the factory site. Typically such load-moving tractor units comprise one or two steerable ground-engaging tractor wheels which can be powered by an on-board motor which could be an internal combustion engine but more generally is an electric motor, either mains powered or battery powered. A coupling including a releasable load hitch is provided between a draw bar of the tractor unit and the load. The coupling provides a substantially rigid connection between the tow bar or the tractor unit and the wheeled load, because the trolleys on which such loads are transported run on castor wheels, and greater steering control can be obtained while moving the load when there is no free movement between the tractor unit and the wheeled trolley supporting the load. In addition the coupling must establish a weight transfer mechanism between the tractor unit and the load in use, so that some of the weight of the load is transferred to the tractor wheels in use. Only in this way is a tractor unit weighing a couple of tens of kilograms able to move a load of up to several tonnes.
GB-A-2279934 discloses one such tractor unit incorporating a screw jack mechanism for transferring some of the weight of the load onto the tractor unit. First the load is coupled, and then the screw jack is actuated manually to transfer the weight of the load onto the tractor unit. Before decoupling can take place the screw jack must again be actuated, this time to remove the load from the tractor unit. This is necessary because when the weight of the load is transferred to the tractor unit that places a very high stress on the coupling hitch, to such an extent that simple unlatching is impossible.
GB-A-1350705 discloses a similar tractor unit in which the coupling hitch between the tractor unit and the load includes a hydraulic lifting ram to engage beneath a relevant part of the load. Actuation of the ram causes a significant portion of the weight of the load to be transferred to the tractor unit. In effect, the lifting ram which is positioned vertically over the driving wheel of the tractor unit attempts to lift the weight of the load or a significant part of the weight of the load onto the tractor wheel.
Tractor units for moving such extremely heavy loads therefore present substantially different problems from those encountered by push/pull tractor units for moving medium sized loads such as supermarket roll cages. Tractor units for such medium sized loads act to lift a coupled end of the load onto the tractor unit where it acts over the tractor wheel or wheels to increase traction, but as a generality the operator himself can lift the load on and off the tractor unit using the leverage of a steering tiller, and there are no excessive forces acting through the coupling hitch. Both the latching together and the unlatching of such a tractor unit and a medium sized load are operations that can be carried out even when the weight of the load acts through the coupling hitch, and such latch engagement/latch release mechanisms can be operated either immediately adjacent to the coupling hitch or from a remote location.
We the inventors have devised a tractor unit for use with heavy loads which avoids the manual or hydraulic transfer of the weight of the load onto the tractor unit prior to using the tractor unit to move the load. The tractor unit can therefore be hitched up to a wheeled load without any of the weight of the load being transferred to the tractor unit. As soon as the tractor unit is driven forwardly or rearwardly relative to the load, however, there is a weight transfer from the load onto the driving wheels of the tractor unit, so that a relatively lightweight tractor unit can move a load of several tonnes. The weight transfer mechanism is described fully below. Furthermore, we have appreciated that when the forward or rearward drive of the tractor unit is terminated in such a tractor unit, the weight of the load is transferred back from the tractor unit so that the coupling hitch between the tractor unit and the load is no longer under such great stress and the load can easily be decoupled in the no-load condition. That opens up the possibility for the first time of having the latch mechanism actuated remotely from a normal operator position as opposed to having to perform the uncoupling operation from adjacent to coupling hitch.
We have also investigated several alternative designs of couplings, but have found that the majority of apparently robust designs of hitch mechanism fail in use when the load being hitched up and moved weighs several tonnes. We have found that the only satisfactory coupling and hitch mechanism is one which comprises telescopically inter-engaging male and female elements. One of those elements is fixed fast to the draw bar of the tractor unit and the other is fixed fast to the load or to a wheeled trolley in which the load is movable. When the male is received telescopically within the female and latched in position there, the coupling has the necessary degree of rigidity and can also transfer the very substantial loads which may be demanded of it.